On arc efficiency in gas tungsten arc welding

On arc efficiency in Gas Tungsten Arc Welding (Sobre eficiência de arco em Soldagem GTAW) Nils Stenbacka University West, Trollhättan, Sweden. Abstract The aim of this study was to review the literature on published arc efficiency values for GTAW and, if possible, propose a narrower band. Articles between the years 1955 – 2011 have been found. Published arc efficiency values for GTAW DCEN show to lie on a wide range, between 0.36 to 0.90. Only a few studies covered DCEP - direct current electrode positive and AC current. Specific information about the reproducibility in calorimetric studies as well as in modeling and simulation studies (considering that both random and systematic errors are small) was scarce. An estimate of the average arc efficiency value for GTAW DCEN indicates that it should be about 0.77. It indicates anyway that the GTAW process with DCEN is an efficient welding method. The arc efficiency is reduced when the arc length is increased. On the other hand, there are conflicting results in the literature as to the influence of arc current and travel speed. Keywords: Gas Tungsten Arc Welding; GTAW; TIG; Arc efficiency; Process parameters; Resumo: O objetivo deste trabalho foi o de fazer uma revisão na literatura de valores publicados de eficiência térmica do arco do processo GTAW e, se possível, propor uma banda mais estreita de valores aceitáveis. Valores publicados para GMAW CC- mostram situar-se numa larga faixa, entre 0,36 a 0,90. Muitos poucos estudos cobrindo CC+ e CA foram encontrados. Informações específicas sobre a reprodutibilidade em estudos calorimétricos tanto quanto em modelagem e simulação (considerando que tanto erros aleatórios como sistemáticos são pequenos) são escassos. Uma estimativa do valor médio da eficiência do arco indica ser este de 0,77. Isto indica, de qualquer forma, que o processo GTAW com CC- é um processo de grande eficiência. A eficiência do arco é reduzida quando o arco é alongado. Por outro lado, existem resultados conflitantes na literatura quando se estuda a influência da corrente de soldagem e velocidade de soldagem. Palavras-Chave: GTAW, TIG, Eficiência de arco, parâmetros de soldagem 1. Introduction Why is it important to know the arc efficiency at TIGwelding more precisely? GTAW is probably the largest arc welding method of all when it comes to welding stainless steels, far larger than for instance GMAW when measured by shielding gas consumption. Of great concern is to know how much of the energy is actually transferred to the material being welded. During the past years a wide range of arc efficiency values for GTAW have been reported in the standard literature, values between 0.22 – 0.80are for instance mentioned [1 - 4]. The intention of this report is to examine background references and the relevance of determined values for GTAW, and if possible to postulate a narrower band for the arc efficiency, ηa. Recebido em 07/10/2013, texto final em 07/10/2013. 380 Arc efficiency (ɳa) (also called process efficiency, thermal efficiency or heat transfer efficiency) plays an important role in many aspects of welding technology. The concept is used for example when estimating process performance, in calculating cooling rates or cooling times, when modeling fusion characteristics etc. In order to accurately utilize modern FEM heat-flow-models it has become more important to know ηa more precisely, and how different process parameters (for example current, welding speed, electrode tip angle and type of shielding gas) will influence the arc efficiency. Arc efficiency ηa is normally defined as; ηa = qw/qn(1) Where qw is the net power input [J/s] transferred to the plate (substrate) and qn is the gross (or nominal) power input at the arc from a certain power source. The part not transferred to the work piece; (1 - ηa), is lost through radiation, convection, electrode heating and heat conduction in the TIG torch. This schematically illustrated by Fig 1. Calorimetric methods are often used to measure qw, and over the years a couple of different Soldag. Insp. São Paulo, Vol. 18, Nº. 04, p.380-390, Out/Dez 2013 On arc efficiency in Gas Tungsten Arc Welding methods have been used which will be commented further on. to note that the analytical 2D model and experimental support for it was given already in 1952 by Wells [21]. With lower travel speeds, heat conduction in the moving direction of the power source will become more important and ηm will consequently be lower. For slow moving power sources, like manual GTAW, the thermal efficiency will indeed be much lower. A value of about 0.16 has for instance been reported by [17] in experiments with slow moving GTAW (travel speed about 0.5 m/min). 2. Methods to determine arc efficiencies Fig 1. TIG torch and the heat transfer around it. The gross energy input during welding can be measured in different ways. In the case of DC arc welding qn is simply the product of current I [A], voltage U [V] at the arc, and welding time t. In the case of a sinusoidal AC current, effective values of voltage u and current i can be used together with the power factor when estimating arc power; P = uicos(φ). A better estimate of the gross arc power, when the current and voltage are highly fluctuating for example in short arc or pulsed arc welding, is to use the arithmetic mean power value. It can be calculated from measured instantaneous current ii and voltage ui values at the arc; Pav = ∑(uiii)/n(2) Summation is made from i =1 to i =n. When the sampling frequency n tends to high values eq. (2) is replaced by; Pav = (1/T) ∫u(t)i(t)dt(3) This physically defines the average power. Integration is made over the time interval T of interest [19]. Melting efficiency ηm(also called fusion efficiency or thermal efficiency of fusion),is another important process property. Process performance can for instance be described by the two parameters ηa and ηm. The arc efficiency (ɳa) must be known in order to experimentally determine the melting efficiency for a given process or a given set of welding parameters. Over the years it has been shown, for various processes, that only a fraction of the effective energy supplied to the substrate is actually used for melting. The remaining part is lost through heat diffusion in the base material. The ratio of the power used for melting (fusion) qm divided by the power actually supplied qw to the substrate defines the melting efficiency according to; ηm = qm/qw(4) Analytical models for calculating ηm have for instance been given by [3-4]. For a rapidly moving high power heat source about 48% of the effective (net) heat input is available for fusion (2D case and no filler material). This is an analytically calculated value based on heat diffusion exclusively perpendicular to the moving heat source and without any addition of filler material [4]. Experiments support this value [14, 16, 21]. It is interesting Soldag. Insp. São Paulo, Vol. 18, Nº. 04, p.380-390, Out/Dez 20 (...truncated)